Fossil fuel powered steam generators, nuclear powered steam systems, and the like, employ many thousands of pipes through which water flows at high temperature and high pressure. The water within the pipe may flow at perhaps 1 m.sup.3 /second, at 600.degree. F. or more, and at pressures of perhaps 1,100 psi. In a nuclear steam system, the outer surface of the pipe may be exposed to liquid at perhaps 2,500 psi. To help stabilize these pipes against vibrations, the pipes are routed through pipe-sized openings in support plates. Unfortunately, the interfaces between these openings and the pipe creates thousands of potential crevices, whose presence can promote pipe cracking.
As used herein, the term "crevice" denotes a water starved region in which heat transfer occurs so rapidly that input water does not arrive sufficiently rapidly to replace water already in the crevice ration that is boiled away. The flowing water is normally non-corrosive, but contains chemical impurities that become concentrated in the crevice as the water boils away. In the water flow-starved boiling region within the crevice, a highly aggressive corrosive brine is created that, unfortunately, can attack, corrode and eventually causes cracks to occur within the pipe wall.
As a result of the corrosion, high temperature and flow pressures, strain within the pipe wall occurs under stress. Cracks, typically forming from the outer wall inward, can result that reduce the strength of the pipe cross-section. Unless detected sufficiently early, pipe failure and costly steam generator down-time can result. The failure mechanism appears to result from progressive intergranular attack and stress corrosion cracking, or "IGA/SCC".
Unfortunately, it is difficult to reliably measure or successfully predict IGA/SCC failure on a long term basis within the hostile environment typified by the steam generators. For example, it is known in the art to monitor strain at high temperatures using non-contact strain measuring techniques. But these devices are poorly suited for long-term measurements on power generation components, whose surfaces are often insulated and/or are subject to degradation. In addition, the pipes to be monitored are frequently located in regions where visual interrogation is difficult or impossible. Further, noncontact strain measurement devices are affected by temperature, opacity, and the turbulence of any intervening atmosphere.
Contact strain gages such as electrical resistance gages are also known in the art. Such devices have long been used to sense strain at temperatures exceeding 700.degree. F. on a long-term basis, and at even higher temperatures for short-term or dynamic measurements. For example, bonded resistance gages are commonly used continuously at temperatures up to 500.degree. F. and have relatively high compliance, e.g., the ability to readily conform to the surface of the object under measurement.
So-called "Eaton" and "Kyowa" weldable resistance gages are also useable at such temperatures, but have less compliance due to package stiffness. In the 600.degree. F. to 650.degree. F. range, such gages are made with a modified nickelchrome alloy that has good drift characteristics, relative small apparent strain, and repeatable apparent strain characteristics. As used herein, "drift" refers to the stability of the strain gage output, while "apparent strain" refers to the change in output of the strain gage as a function of temperature in a regime in which hysteresis effects do not predominate. It is difficult to adequately temperature compensate such devices using heat treatment techniques. Above 650.degree. F., the sense material undergoes a metallurgical phase transition that can "reset" the temperature compensation, causing radical zero shifts.
In the 1000.degree. F. to 1100.degree. F. range, it is difficult to retain calibration, especially with resistance strain gages. Apparent strain, drift, and hystersis due to temperature cycling present problems. At present, it is not known how to accomplish long-term static strain measurement at such elevated temperatures.
The drift problem has been somewhat addressed in the prior art using high-temperature capacitive strain gages. However, such devices are not generally suitable for dynamic measurements above 100 Hz. Although low drift characteristics enable capacitive strain gages to measure creep strain change at steady-state, installation, calibration and other documentation is costly. Nonetheless, capacitive type gages represent the only presently available contact devices useable for field measurement of creep strain at temperatures above 1000.degree. F.
All of the above-described gages suffer the common problem of requiring electrical connections, which frequently are difficult to implement in a power plant generator environment. By contrast, fiber optic strain gages do not require such connections, and are useable at temperatures exceeding 700.degree. F.
A microbend fiber optic type strain gage is described in U.S. Pat. No. 5,020,379 to Berthold, in which a strain sensing optical fiber is sandwiched between a pair of tooth-edged end plates. When the end plates move toward or away from each other, the fiber is deformed, amplitude modulating a light signal transmitted through the fiber. This modulation is detected to provide strain information having excellent resolution. A second, reference, optical fiber is sandwiched between a similar pair of tooth-edged end plates that are locked to each other but not attached to the structure. Both fibers are equal in length and are routed in parallel. As such, the reference fiber compensates for source brightness variations and changes in fiber transmission over time, thus providing temperature compensation.
As such, the Berthold device is similar to a conventional strain gage in that it modulates the "resistance" to light passing through the sensing fiber. The device is also similar to a conventional capacitance gage in that it relies upon relative movement of two plates for its measurement. However, unlike the capacitance gage, the microbend fiber stiffness makes the gage less compliant than capacitance gages, but still more compliant than weldable resistance gages. The microbend fiber optic strain gage can provide stable and extended life data at temperatures up to 1100.degree. F.
In short, although a variety of strain gages are known in the prior art, there remains a need for a mechanism by which the onset and progression of IGA/SCC can be predicted on a long term basis in the hostile environment that typifies power generation plants.
The present invention provides a method and apparatus for providing such predictions.